Currently, navigation devices of moving bodies such as cars are known. This navigation device displays a vehicle position on a map, and gives a guidance to a destination. When a vehicle position is displayed on a road on a map, a GPS positioning device which obtains GPS satellite positioning results and various sensors such as a velocity sensor, an angular velocity sensor and an acceleration sensor are used to observed and measure a vehicle motion and then processing called map matching is performed to identify the vehicle position on a road link of map data.
However, a moving distance, a yaw angle or a pitch angle of the vehicle measured by the sensors have a measurement error. Therefore errors (measurement errors) gradually accumulate according to autonomous navigation which adds moving vectors indicated by the sensors. An error accumulated in this way is optionally corrected using a GPS position (a vehicle position calculated by the GPS receiver) and a GPS azimuth (a vehicle azimuth calculated by the GPS receiver) observed by the GPS receiver provided independently from the sensors.
Hereinafter, a GPS positioning principle will be described. According to GPS positioning, a vehicle position (unknown value) is three-dimensionally calculated using positions (known values) of three or more GPS satellites and distances (known values) between these GPS satellites and the vehicle based on a principle of triangulation. Each GPS satellite and a vehicle make independent motions, and therefore times need to be synchronized with a common time sequence (also referred to as a GPS Time below). Hence, four unknown values including vehicle positions (x, y, z) and a built-in clock error are calculated using four or more GPS satellites of the three GPS satellites plus one GPS satellite. By synchronizing the times, it is possible to calculate distances (pseudo distances [m]) between the GPS satellites and the vehicle based on radio wave propagation times which pass until radio waves transmitted from the GPS satellites are received by the vehicle.
Further, carrier frequencies of the radio waves transmitted from the GPS satellites Doppler-shift based on relative motions between the GPS satellites and the vehicle. Therefore, the vehicle velocities (vgx, vgy, vgz) are three-dimensionally calculated based on the shift amount of the Doppler-shifted frequency ([Hz]), and a vehicle azimuth is calculated based on the calculated vehicle velocities. A range rate ([m/s]) converted from the shift amount of the Doppler-shifted frequency indicates a time change amount of pseudo distances (delta range ([m/s])). However, it is necessary to estimate a range rate in case that the vehicle is assumed to be stopping to calculate a vehicle velocity.
To correct the vehicle position and azimuth using the GPS position and the GPS azimuth, it is necessary to taken into account the following problems related to GPS positioning.
(1) There is a problem that, when radio waves of GPS satellites above the vehicle (own vehicle) are shielded by architectures around the vehicle and radio waves of only three GPS satellites can be received, only three unknown values can be calculated, and therefore when the number of GPS satellites from which radio waves can be received is less than three, the vehicle position and velocity cannot be observed.
(2) When radio waves transmitted from GPS satellites and reflected by architectures around a vehicle are received, errors (multipaths) are produced in radio wave propagation times (or pseudo distances), and therefore GPS positioning accuracy lowers. More specifically, when a multipath is occurred, a trajectory shape of a GPS satellite is distorted, or even when a trajectory shape is good, a vehicle position deviation or azimuth deviation temporarily occurs.
(3) Lower speed driving which decreases a difference between a measurement value of a range rate due to a Doppler shift and an estimated value of a range rate in case that a vehicle is assumed to be stopping makes larger a rate of an error included in a GPS velocity (vehicle velocity calculated by the GPS receiver). When the GPS velocity error is greater, a GPS azimuth error calculated by the GPS velocity is also greater.
(4) Clocks in both of GPS satellites and a vehicle drift (change at an order of ns), and therefore it is necessary to correct the respective clocks. Expensive atomic clocks of little drifts are used for clocks to be mounted on GPS satellites, and error compensation parameters of the atomic clocks are broadcast (transmitted) from the GPS satellites. Therefore, the vehicle can correct the errors of clocks mounted on the GPS satellites by receiving radio waves transmitted from the GPS satellites. Meanwhile, a crystal clock of a significant drift is used for a clock mounted on a vehicle (also referred to as a built-in clock), and there is no error compensation parameter. Hence, an error of a built-in clock (also referred to as a built-in clock error) is calculated and corrected upon calculation of GPS position. However, even after correction, an error equal to or less than 1 μsec is left in the built-in clock. Such a built-in clock error becomes a measurement error of a range rate common to all reception satellites.
(5) Indexes accurately indicating positioning errors (a position error, a velocity error and an azimuth error) are not outputted from a GPS receiver.
A conventional navigation device devises a method of evaluating GPS positioning accuracy and correcting a vehicle position (own vehicle position) to increase precision of the own vehicle position (see Patent Documents 1 and 2).
According to, for example, Patent Document 1, an autonomous position (a vehicle position calculated by sensors) is corrected on a road link per predetermined time or predetermined distance. When map matching cannot be performed by autonomous navigation, an autonomous position is optionally corrected based on a good GPS positioning result. More specifically, last trajectories (positions and azimuths) of a GPS position and an autonomous position in a predetermined zone are stored per predetermined time or predetermined distance. Reliability of GPS is accredited based on a difference between total sums of moving vectors of the respective positions which configure the respective trajectories. Then, a threshold (GPS error circle) for correcting an autonomous position based on the GPS position is set. When the GPS trajectory and an autonomous trajectory substantially match, a difference between positions configuring respective trajectories is little. When a GPS positioning state is poor, the GPS trajectory and the autonomous trajectory differ, and the difference between positions configuring the respective trajectories is great. Reliability of a GPS positioning result is determined based on these characteristics. When the reliability is low, a high threshold is set, and, when a difference between a GPS position and an autonomous position is greater than the threshold, the autonomous position is corrected based on the GPS position. In addition, the above predetermined zone may be last 200 m or may be last 10 seconds to 15 seconds.
Further, an object of Patent Document 2 is to increase accuracy of an autonomous position. An autonomous trajectory is corrected such that a difference between a trajectory of higher reliability among a GPS trajectory (a vehicle trajectory calculated based on information received from GPS satellites) and a matching trajectory (a vehicle trajectory calculated by map matching processing), and an autonomous trajectory (a vehicle trajectory calculated by autonomous navigation) decreases. As to a trajectory based on which an autonomous position is corrected, the GPS trajectory is selected when the GPS trajectory is highly reliable and accurate, and a matching trajectory is selected when the GPS trajectory has reliability equal to or less than a predetermined value. The reliability of the GPS trajectory is created based on a result of comparison between the autonomous trajectory and the GPS trajectory. According to map matching, when an own vehicle position is identified on a road link on which an autonomous trajectory and a road link shape match, matching reliability is created by comparing the matching trajectory and the autonomous trajectory. The less a road width of a road link on which the own vehicle position is identified is narrower and fluctuation of a vehicle azimuth is, the higher reliability of a matching trajectory is. In addition, an autonomous azimuth is also corrected by the same method as the method of correcting the autonomous position.